C J Lister

Gamma-ray spectroscopy experiments for levels in 39K below 8.4 MeV excitation

The gamma-ray spectroscopy of sixty excited states in 39K below an excitation energy of 8.4 MeV has been studied. The data were obtained mainly from the 36Ar+ alpha reaction at beam energies between 7.5 and 16.0 MeV, the remaining data coming from the 24Mg+18O and 28Si+16O reactions, both at beam energies of 42 MeV. 13 new levels were established in 39K and several previously unobserved branches were revealed between the previously known levels. Spins, parities, mixing ratios and gamma-ray branching ratios were determined from angular distribution and linear polarisation measurements using the 36Ar+ alpha reaction at several different beam energies. Mean lifetimes were deduced using both the Doppler-shift attenuation method (with the 36Ar+ alpha reaction) and the recoil-distance method (with the 28Si+16O reaction). The results have allowed transition strengths to be calculated for the decays between many of the levels. The results are discussed in terms of both the shell model and the simple weak coupling model.

The structure of levels in 67Ga below 2.5 MeV excitation

The reaction 64Zn( alpha ,p gamma )67Ga was used to populate levels of 67Ga. Excitation energies, mean lifetimes, gamma -ray branching ratios, angular distributions and linear polarisations have been measured for levels below Ex=2.5 MeV. Gamma ray spectra at bombarding energies in the range Ex=7-16 MeV and gamma - gamma coincidence data at Ealpha =13.0 and 15.0 MeV were used to determine the decay scheme.

Gamma-ray spectroscopy of high-spin levels in 67Ga

Reactions 64Zn( alpha ,p gamma )67Ga and 53Cr(16O, pn gamma )67Ga populated the levels in 67Ga. Gamma-gamma coincidence experiments were carried out at Ealpha =13 and 15 MeV with the ( alpha ,p gamma ) reaction and E(16)O=42 MeV with the heavy-ion reaction. The Doppler-shift attenuation method was used with the ( alpha ,p gamma ) reaction to deduce mean lifetimes (or limits) for certain levels, along with the recoil-distance method utilised both in singles and gamma - gamma coincidence with the heavy-ion reaction. A mean lifetime of tau m=60+or-30 ns for the first excited state at 167 keV was measured using the delayed coincidence method. Gamma-ray angular distributions and linear polarisations were measured at Ealpha =13.5 MeV using a Ge(Li)-NaI(Tl) escape-suppressed spectrometer and a three-Ge(Li) Compton polarimeter and analysis resulted in spin and parity assignments. Transition strengths were calculated and the results are discussed in terms of the particle-cluster-vibrational-core coupling model.

Decay modes and level lifetimes in 41Ca and 41K

Levels in 41Ca and 41K were populated using the reactions 38Ar( alpha ,n)41Ca and 38Ar( alpha ,p)41K. Solid 38Ar targets were produced using a 4He refrigerator system at a mean temperature of 14K. Gamma-gamma coincidence data were collected at Ealpha =10.50, 11.50 and 15.00 MeV to study principal decay modes. In 41Ca new gamma decays from levels at 3975 and 4519 keV were found. Mean lifetimes were measured using the Doppler-shift attenuation method at several beam energies. Data were obtained for 36 levels in 41Ca and 21 levels in 41K.

The structure of high-spin positive-parity states in 43Ca

Abstract The 40Ar(α, nγ)43Ca reaction has been used to study high-spin states in 43Ca. The γ-ray decays of levels at 3505 and 4186 keV are established in γ-γ coincidence measurements and new decays are observed from levels at 3944 and 4591 keV. Solid Ar targets enable the Doppler shift attenuation method to be used to measure mean lifetimes for the following levels E x ( keV )(τ m )J π : 2951(>5 ps ) 11 2 + ; 3371(>5 ps ) 13 2 + ; 3505 (105±35 fs ) 13 2 + ; 3944 (1.1±0.3 ps ) 15 2 + ; 4186 (180±70 fs ) 15 2 + and 4591 (300±75 fs ) ( 13 2 + , 17 2 + ) . The structure of these levels is discussed in terms of a deformed K π = 3 2 + band and shell model configurations of the type ( f 7 2 ) 7 + 4 ⊗(d 3 2 ) −1 .

High spin states in 39K

The reaction 36Ar( alpha , p gamma )39K has been used to measure angular distributions, linear polarizations, mixing ratios, branching ratios and mean lifetimes for a number of high spin states in 39K. The results give the following unambiguous spin and parity assignments: ExJpi ; 2814 keV 7/2-; 3597 keV 9/2-; 3944 keV 11/2-; 5718 keV 9/2- and 6475 keV 11/2+. Using a solid 36Ar target and the Doppler shift attenuation method a mean lifetime of 250+or-65 fs has been measured for the 5718 keV level. There are several discrepancies between the present results and the results of previous work, particularly the heavy ion data published by the Brookhaven group.

Angular distribution and linear polarisation measurements of gamma rays in 41Ca

The use of the38Ar( alpha ,n gamma )41Ca reaction on a frozen target, together with high-precision gamma-ray measurements, has allowed many rigorous spin and parity measurements to be made on states in 41Ca. Several restrictions of spin and parity were possible and many multipole mixing ratios were deduced. Simple phenomenological models are used to give a first-order description of most states of both positive and negative parity up to 4 MeV in excitation. Collective core excitations are found to have a strong influence on the properties of many states.

The production and use of solid targets from gaseous elements

Abstract A method for producing solid targets from materials which are normally gaseous at room temperature is described. The system, which is based on a closed cycle helium refregerator capable of a lowest temperature of 10 K, can be used for all gases except H, He and Ne. The apparatus has bee designed such that targets a few mg cn −2 thick can be produced with very described, together with some examples of the use of isotopically enriched solid argon targets.

A comprehensive study of the γ-ray spectroscopy of 43Ca

The reaction 40Ar( alpha ,n gamma )43Ca was used to populate the levels in 43Ca at bombarding energies between 5.5 and 19 MeV, gamma - gamma coincidence measurements at Ealpha =8.5, 15 and 16 MeV being used to establish the decay scheme of 53 levels. Several new levels and new decays of levels with high angular momentum were established. The Doppler-shift attenuation method was used with solid Ar targets to deduce the mean lifetimes for most of these levels. A Ge(Li)-Na(Tl) escape-suppressed spectrometer and a three-Ge(Li) Compton polarimeter were used to measure the angular distribution and linear polarisation of gamma rays at alpha bombarding energies of 7.0, 14.5 and 17 MeV.

Decay scheme of excited states in 67Ge

Excited states in 67Ge were studied using the reaction 64Zn( alpha ,n gamma ). Excitation functions for the production of 67Ge determined the Q value for this reaction to be -8993+or-5 keV. Neutron-gamma coincidences at Ealpha =15 MeV and gamma - gamma coincidences at Ealpha =13 and 15 MeV helped in the placing of 25 gamma rays into a decay scheme of 14 levels in 67Ge. The first excited state was placed at 18.3 keV and the 734 keV gamma ray has been identified as a decay from a level at 751.8 keV to the 18.3 keV level. The mean lifetimes of the isomeric states at 18.3 keV ( tau m=21.0+or-0.3 mu s) and 751.8 keV ( tau m=160+or-2 ns) have been determined by pulsed-beam techniques, the decay of the 18.3 keV state being detected by X-ray detection. The mean lifetime of the level at 751.8 keV (Jpi =9/2+) and the angular distribution and linear polarisation measured for the 734 keV gamma ray have limited the spin of the level at 1803 keV to 5/2 with a proposed negative parity.